Biogeochemical constraints on the origin of methane in an alluvial aquifer: evidence for the upward migration of methane from underlying coal measures

Abstract. Geochemical and microbiological indicators of methane (CH 4 ) production, oxidation and migration processes in groundwater are important to understand when attributing sources of gas. The processes controlling the natural occurrence of CH 4 in groundwater must be understood, especially when considering the potential impacts of the global expansion of coal seam gas (CSG) production on groundwater quality and quantity. We use geochemical and microbiological data, along with measurements of CH 4 isotopic composition ( δ 13 C-CH 4 ), to determine the processes acting upon CH 4 in a freshwater alluvial aquifer that directly overlies coal measures targeted for CSG production in Australia. Measurements of CH 4 indicate that there is biogenic CH 4 in the aquifer; however, microbial data indicate that there are no methanogenic archaea in the groundwater. In addition, geochemical data, particularly the isotopes of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC), as well as the concentration of SO 4 2− , indicate limited potential for methanogenesis in situ. Microbial community analysis also shows that aerobic oxidation of CH 4 occurs in the alluvial aquifer. The combination of microbiological and geochemical indicators suggests that the most likely source of CH 4 , where it was present in the freshwater aquifer, is the upward migration of CH 4 from the underlying coal measures.